During combustion in a fluidized bed, the fluidized bed is supplied with air for fluidization of the bed material and for combustion of fuel supplied to the fluidized bed. If the fluidized bed is part of a plant for combustion in a pressurized fluidized bed, a PFBC plant, the fluidized bed contained within a bed vessel is enclosed in a pressure vessel and the air supplied to the fluidized bed is pressurized, for example in a compressor driven by a gas turbine.
The mass flow of pressurized air supplied to a PFBC plant is controlled within an interval of 40-105% of nominal flow. The pressurization is normally carried out in a gas turbine-driven compressor. From the point of view of capital cost, high compression ratios are desirable A gas turbine-driven compressor provides different possibilities of controlling the mass flow, depending on the type of gas turbine. A single-shaft unit may control the mass flow by varying the adjustment of compressor guide vanes and inlet valves, and, in addition, compressed air may be recirculated through the compressor. Moreover, in a multi-shaft unit, adjustable turbine guide vanes and nozzles as well as variable rotor speed are utilized.
The temperature of the air supplied from the compressor via the pressure vessel to the fluidized bed must be limited, both when the air is used for cooling of pressure vessel, bed vessel, cyclones and other supporting components arranged in the pressure vessel, and when temperature variations, caused by compression ratios and ambient temperature, in air supplied to the fluidized bed affect the output power from the plant and the efficiency of the plant.
The temperature of air supplied to the pressure vessel is not limited in normal PFBC plants, and thus there is no equalization of the temperature variations which occur in the pressurized air. Temperature variations occur as a consequence of variations in the ambient temperature and varying compression ratios and are compensated for in a normal PFBC plant by a change in the output power from the plant and in the efficiency of the plant.
The residual heat in flue gases emitted from a combustion plant is delivered to flue gas economizers, which are arranged in the flue gas paths.